[ELF2] Sort dynamic symbols according to the MIPS requirements.

[ikudrin]

Summary

The MIPS ABI has its requirements to sort entries in the .dyn.sym section.
Symbols, which are not in the GOT, have to precede symbols added in the GOT.
These latter must have the same order as the corresponding GOT entries.

Since these sorting requirements contradict those of the GNU hash section,
they cannot be used together.

[ruiu]

ELF/OutputSections.cpp

925–926 ↗

(On Diff #39047)

This assert is probably too much, so remove that.

933–938 ↗

(On Diff #39047)

Can you make this a separate non-member function so that we can write

std::stable_sort(Symbols.begin(), Symbols.end(), sortMipsSymbols<ELFT>);

[ikudrin]

• Remove the assert.
• Make a separate function to sort symbols.

[• rafael]

ELF/OutputSections.cpp

917 ↗

(On Diff #39349)

Can you include a llink (and section number) to the ABI?

918 ↗

(On Diff #39349)

Why do you need the template?

[ikudrin]

• Remove redundant template.
• Add link to the documentation.

[ikudrin]

• Rebase on the top (because D14450 depends on this revision as well as the latest commits).

[atanasyan]

LGTM

[ruiu]

LGTM

lld/trunk/ELF/Driver.cpp

for (auto *Arg : Args) {
     case OPT_no_whole_archive:
       WholeArchive = false;
       break;
     }
   }
 
   if (Files.empty())
     error("no input files.");
+
+  if (Config->GnuHash && Config->EMachine == EM_MIPS)
+    error("The .gnu.hash section is not compatible with the MIPS target.");
 }
 
 template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   SymbolTable<ELFT> Symtab;
   Target.reset(createTarget());
 
   if (!Config->Shared) {
     // Add entry symbol.

lld/trunk/ELF/OutputSections.cpp

: OutputSectionBase<ELFT>(
       Table(Table), StrTabSec(StrTabSec) {
   typedef OutputSectionBase<ELFT> Base;
   typename Base::Elf_Shdr &Header = this->Header;
 
   Header.sh_entsize = sizeof(Elf_Sym);
   Header.sh_addralign = ELFT::Is64Bits ? 8 : 4;
 }
 
+// Orders symbols according to their positions in the GOT,
+// in compliance with MIPS ABI rules.
+// See "Global Offset Table" in Chapter 5 in the following document
+// for detailed description:
+// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
+static bool sortMipsSymbols(SymbolBody *L, SymbolBody *R) {
+  if (!L->isInGot() || !R->isInGot())
+    return R->isInGot();
+  return L->GotIndex < R->GotIndex;
+}
+
 template <class ELFT> void SymbolTableSection<ELFT>::finalize() {
   if (this->Header.sh_size)
     return; // Already finalized.
 
   this->Header.sh_size = getNumSymbols() * sizeof(Elf_Sym);
   this->Header.sh_link = StrTabSec.SectionIndex;
   this->Header.sh_info = NumLocals + 1;
 
   if (!StrTabSec.isDynamic()) {
     std::stable_sort(Symbols.begin(), Symbols.end(),
                      [](SymbolBody *L, SymbolBody *R) {
                        return getSymbolBinding(L) == STB_LOCAL &&
                               getSymbolBinding(R) != STB_LOCAL;
                      });
     return;
   }
   if (Out<ELFT>::GnuHashTab)
     // NB: It also sorts Symbols to meet the GNU hash table requirements.
     Out<ELFT>::GnuHashTab->addSymbols(Symbols);
+  else if (Config->EMachine == EM_MIPS)
+    std::stable_sort(Symbols.begin(), Symbols.end(), sortMipsSymbols);
   size_t I = 0;
   for (SymbolBody *B : Symbols)
     B->setDynamicSymbolTableIndex(++I);
 }
 
 template <class ELFT>
 void SymbolTableSection<ELFT>::addLocalSymbol(StringRef Name) {
   StrTabSec.add(Name);

lld/trunk/test/elf2/mips-dynsym-sort.s

+# Check the order of dynamic symbols for the MIPS target.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t-be.o
+# RUN: ld.lld2 -shared %t-be.o -o %t-be.so
+# RUN: llvm-readobj -symbols -dyn-symbols %t-be.so | FileCheck %s
+
+# RUN: llvm-mc -filetype=obj -triple=mipsel-unknown-linux %s -o %t-el.o
+# RUN: ld.lld2 -shared %t-el.o -o %t-el.so
+# RUN: llvm-readobj -symbols -dyn-symbols %t-el.so | FileCheck %s
+
+# REQUIRES: mips
+
+  .data
+  .globl v1,v2,v3
+v1:
+  .space 4
+v2:
+  .space 4
+v3:
+  .space 4
+
+  .text
+  .globl  __start
+__start:
+  lui $2, %got(v3) # v3 will precede v1 in the GOT
+  lui $2, %got(v1)
+
+# Since all these symbols have global binding,
+# the Symbols section contains them in the original order.
+# CHECK: Symbols [
+# CHECK:     Name: v1
+# CHECK:     Name: v2
+# CHECK:     Name: v3
+# CHECK: ]
+
+# The symbols in the DynamicSymbols section are sorted in compliance with
+# the MIPS rules. v2 comes first as it is not in the GOT.
+# v1 and v3 are sorted according to their order in the GOT.
+# CHECK: DynamicSymbols [
+# CHECK:     Name: v2@
+# CHECK:     Name: v3@
+# CHECK:     Name: v1@
+# CHECK: ]

lld/trunk/test/elf2/mips-gnu-hash.s

+# Shouldn't allow the GNU hash style to be selected with the MIPS target.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t-be.o
+# RUN: not ld.lld2 -shared -hash-style=gnu %t-be.o -o %t-be.so 2>&1 | FileCheck %s
+
+# RUN: llvm-mc -filetype=obj -triple=mipsel-unknown-linux %s -o %t-el.o
+# RUN: not ld.lld2 -shared -hash-style=gnu %t-el.o -o %t-el.so 2>&1 | FileCheck %s
+
+# CHECK: The .gnu.hash section is not compatible with the MIPS target.
+
+# REQUIRES: mips
+
+  .globl  __start
+__start:
+  nop